Notifications

Log in to see your notifications.

Reproduction

12 min read

The human reproductive system

In sexual reproduction a male gamete (sperm) fuses with a female gamete (egg) in fertilisation to form a zygote. The reproductive organs make the gametes and the hormones that control them.

The male reproductive systemThe testes make sperm and the hormone testosterone and hang in the scrotum, which keeps them slightly cooler than the body. Sperm mature in the epididymis, pass along the sperm duct, and mix with fluid from the seminal vesicle and prostate gland. They leave the body through the urethra in the penis.bladdertestis (makes sperm)scrotum + testissperm ductseminal vesicleprostate glandurethra / penisepididymis
The female reproductive systemTwo ovaries make eggs and the hormones oestrogen and progesterone. Each month an egg is released into the oviduct, where it may be fertilised by a sperm. The fertilised egg travels to the uterus, whose wall has thickened, and implants. The cervix at the lower end of the uterus opens into the vagina.ovary (makes eggs)oviduct (egg tube)uterus (womb)cervixvaginaovary

The menstrual cycle

From puberty to the menopause a woman has a cycle of about 28 days. An egg is released from an ovary at ovulation, about halfway through the cycle. The wall of the uterus thickens to receive a fertilised egg; if there is no pregnancy the wall breaks down and is lost in menstruation.

The menstrual cycleA 28-day cycle. Menstruation lasts about the first five days. FSH from the pituitary gland makes a follicle in the ovary grow and release oestrogen, which rebuilds the uterus lining. A surge of LH on about day 14 causes ovulation. The empty follicle makes progesterone, which keeps the lining thick; if there is no pregnancy the hormones fall and the lining is shed.day of the cycle (28 days)hormone leveloestrogenprogesteroneLH surgeovulation (day 14)periodlining rebuildsthick lining (ready for an embryo)
The chance of fertilisation is greatest around ovulation, on about day 14 of the cycle.

Pregnancy and birth

A fertilised egg divides and implants in the uterus wall. The embryo grows into a foetus, which is fed through the placenta and cord. Pregnancy lasts about 40 weeks. At birth, strong contractions push the baby out through the vagina.

The placentaThe placenta joins the foetus to the wall of the uterus through the umbilical cord. The mother’s blood and the foetal blood come very close but do not mix. Oxygen and food pass from mother to foetus, and carbon dioxide and urea pass from foetus to mother. The amnion and amniotic fluid protect the foetus.amniotic fluidfoetusumbilical cordmother's blood inblood outto foetus: oxygen, food, antibodiesto mother: carbon dioxide, urea

Sexually transmitted infections

Infections such as gonorrhoea, syphilis and HIV are spread by sexual contact. They are prevented by abstinence, faithfulness to one uninfected partner, and condoms, and treated early with medicine.

Try itFertile window

Choose a day of the 28-day cycle to see how likely fertilisation would be.

Check yourself Where does fertilisation normally take place?
Check yourself What is ovulation?
Check yourself What crosses the placenta from mother to foetus?

Key terms

Gamete
A sex cell: sperm or egg.
Zygote
The cell formed by fertilisation.
Ovulation
Release of an egg from an ovary.
Placenta
The organ that feeds the foetus and removes its wastes.

Summary

  • Sperm and egg fuse in fertilisation to form a zygote.
  • The menstrual cycle lasts about 28 days with ovulation around day 14.
  • The foetus is fed through the placenta; the blood does not mix.
  • STIs are prevented by abstinence, faithfulness and condoms.
Regulation